It can be foreseen that the use of a metal oxide semiconductor (MOS) structure as a basic switching device will reach its limit, as device packing density increases. In a case of the MOS structure with a device packing density beyond a 4 gigabyte (GB) dynamic random access memory (DRAM) range, a switching operation using a gate voltage in accordance with the principles of a MOS device operation will be impossible, because the distance between a source and a drain becomes too close. In other words, integration of the MOS structure is typically limited to about 4 gigabyte DRAM because malfunctions of the device may occur due to tunneling between the source and the drain and through a gate oxide film even when no gate voltage is applied. Therefore, to fabricate a device of gigabyte or terabyte class, a form of device other than the current MOS structure should be employed. The form of device suggested by many research groups is the single electron transistor (SET).
The SET is a device employing the so-called Coulomb blockade effect pertaining to quantum effects exhibited by the interaction between electrons having a nano-scale dimension, whereby any further tunneling of charges is suppressed during the process of tunneling charge carriers such as electrons or holes through an insulation film, such that the individual flow of discrete electrons can be controlled.
The principle of the Coulomb blockade effect caused by an SET tunneling is as follows. If the total capacitance caused by a region into which electrons enter through tunneling is very small, a charge effect of the discrete electrons can be observed. If a charge energy e2/2C of the discrete electron charge is greater than an energy kBT of a thermal vibration and there is no voltage increase applied externally when the temperature remains constant, an electron can not have the energy required for charging a capacitor by tunneling. Accordingly, no further tunneling occurs once one electron is charged. That is, once an electron previously tunneled and charged in a capacitor causes a low voltage having a level of at least a voltage drop at the capacitor is applied to the next electron, the next electron does not achieve the level of energy required for charging by tunneling, and thus no further tunneling occurs. This effect of suppression of further tunneling due to electrons that have already tunneled called a Coulomb blockade effect.
Generally, the SET structure using the Coulomb blockade effect has, like a MOS structure including a source, a drain, and a gate, with a channel having conductive quantum dots for facilitating a discrete electron flow. Therefore, the channel consists of an insulating material and the conductive quantum dots, facilitating electron flow by using discrete tunneling.
To fabricate a nano device using the quantum dots, technology enabling formation of the quantum dot with the crystal characteristics of a single crystal type and the technology for forming a delicate and uniform quantum dot are required.
The conventional technologies in forming the quantum dots are the method of using lithography, and the method of depositing SiO2 while making a high ratio of silicon to form a silicon quantum dot.